Moving laser beam readers or laser scanners, as well as solid-state imaging systems or imaging readers, have been used, in both handheld and/or hands-free modes of operation, to electro-optically read targets, such as one- and two-dimensional bar code symbols, each bearing elements, e.g., bars and spaces, of different widths and reflectivities, to be decoded, as well as non-symbol targets or forms, such as documents, labels, receipts, signatures, drivers' licenses, employee badges, and payment/loyalty cards, each bearing alphanumeric characters, to be imaged.
The moving laser beam reader generally includes a laser for emitting a laser beam, a focusing lens assembly for focusing the laser beam to form a beam spot having a certain size at a focal plane in a range of working distances, a scan component for repetitively scanning the beam spot across a target in a scan pattern, for example, a scan line or a series of scan lines, across the target multiple times per second, e.g., forty times per second, a photodetector for detecting laser light reflected and/or scattered from the target and for converting the detected laser light into an analog electrical signal, and signal processing circuitry including a digitizer for digitizing the analog signal, and a microprocessor for decoding the digitized signal. The digitized signal is then used to identify the target.
The imaging reader includes a solid-state imager or image sensor having an array of cells or photosensors that correspond to image elements or pixels in a field of view of the image sensor, an aiming light assembly having an aiming light source, e.g., an aiming laser, and an aiming lens for generating an aiming light pattern or mark on a target prior to reading, an illuminating light assembly for illuminating the field of view with illumination light from an illumination light source, e.g., one or more light emitting diodes (LEDs), and an imaging lens assembly for capturing return ambient and/or illumination light scattered and/or reflected from the target being imaged over a range of working distances and for projecting the captured light onto the array. Such an image sensor may include a one- or two-dimensional charge coupled device (CCD) or a complementary metal oxide semiconductor (CMOS) device and associated circuits for producing electronic signals corresponding to a one- or two-dimensional array of pixel information over the field of view.
As advantageous as both types of electro-optical readers have been in reading targets, it is always desirable to enhance performance. Increasing the intensity or brightness of the laser beam of the laser in the moving laser beam reader will increase the working distance range, because there will be correspondingly more return light to detect from targets that are further away from the moving laser beam reader. Similarly, increasing the intensity or brightness of the aiming laser in the imaging reader will increase performance, because the aiming pattern will be more visible to an operator, especially for targets that are further away from the imaging reader.
However, increasing the laser beam intensity too much for either the laser in the moving beam reader or the aiming laser in the imaging reader may violate human eye exposure laser safety standard limits. For example, a class 2 laser is limited to an output power of 1 mW over a base time interval of 250 msec, and a class 1 laser is limited to an output power of 0.39 mW over a base time interval of 10 sec. The laser beam intensity cannot exceed these limits not only in normal operation, but also in the event of reader malfunction or failure of laser power control circuitry specifically provided in each reader to insure that these limits are never surpassed.
The known laser power control circuitry in such readers monitored the laser current in order to provide feedback about the output power of the laser beam. Also, an internal light detector, e.g., a semiconductor monitor photodiode, was typically mounted inside the laser adjacent a semiconductor laser chip, for monitoring the output power of the laser beam. A microprocessor or programmed controller was operatively connected to the monitor photodiode, for controlling a monitored output power of the laser beam by deenergizing the laser when the monitored output power of the laser beam exceeds a safe power level limit.
For example, U.S. Pat. No. 7,609,736 disclosed a laser power control arrangement, in which power to such a laser was interrupted upon detection of an over-power condition not conforming to preestablished regulatory standards. During an operational mode, a difference between laser drive currents at two operating points was compared to a difference between laser drive currents at the same two operating points during a calibration mode. A programmed controller set the operating points by adjusting a digital potentiometer to different potentiometer settings. The over-power condition was recognized when the difference during the operational mode exceeded the difference during the calibration mode by a predetermined amount.
As advantageous as the known laser power control arrangements have been in regulating laser output power, there are special circumstances in which the laser safety standard limits could still be exceeded. For example, the monitor photodiode could become disconnected and, without corrective feedback, the laser chip could be driven with a very large amplitude drive current. As another example, a gate of a drive transistor that supplies the drive current to the laser could short, and turn the drive transistor fully on, thereby again driving the laser chip with a very large amplitude drive current. In the prior art, such a large amplitude drive current would typically cause the laser chip to burn out and be instantly destroyed. Although the laser is destroyed, at least there is compliance with the existing governmental regulatory safety standard for a laser.
However, modern lasers are typically more robust than those employed in the prior art and can sustain higher drive currents without burning out. In such circumstances, despite the presence of the known laser power control arrangements, the modern lasers are not destroyed and, when driven at the higher drive currents, continue to produce laser output powers that might exceed the safety standard limits. The difference between older and more recent lasers can mean the difference between compliance and non-compliance with the laser safety standard.
Accordingly, there is a need for an arrangement for, and a method of, regulating laser output power in electro-optical readers that complies with the governmental regulatory laser safety standards, despite the presence of high drive currents that do not cause destruction of the lasers.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions and locations of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.
The arrangement and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.